1. Field of the Invention
The present invention relates to improvement in a panel display apparatus.
2. Description of the Prior Art
FIG. 5 shows a conventional panel display apparatus such as a liquid crystal display. The panel display apparatus of FIG. 5 designed to display images on both a CRT (Cathode-Ray Tube) 5 and a panel display 7, comprises a CPU (Central Processing Unit) 1, a V-RAM (Video Random Access Memory) 2, a display controller 3, and the like. A video signal read from the V-RAM 2 is outputted by the display controller 3 in accordance with instructions of the CPU 1 as a few bit digital video signals VD.sub.o -VD.sub.k. The digital video signals VD.sub.o -VD.sub.k are converted into an analog video signal by a D/A (Digital-to-Analog) converter 4 and fed into the CRT 5. Then the CRT 5 produces an image in accordance with both the resulting analog video signal and horizontal/vertical synchronizing signals H/V that are fed by the display controller 3.
The digital video signals VD.sub.o -VD.sub.k outputted by the display controller 3 are also fed into a panel display unit 6. Then the panel display unit 6 produces an image in accordance with both the digital video signals VD.sub.o -VD.sub.k and the aforementioned horizontal/vertical synchronizing signals H/V. Thus, using the same digital video signals VD.sub.o -VD.sub.k, both the CRT 5 and the panel display 7 simultaneously reproduce the same image.
However, the conventional panel display apparatus as described above has the following problems.
Because the pixels in the panel display 7 are thoroughly independent of each other, an image reproduced on the panel display 7 can be represented as an aggregate of images in the units of pixels obtained from the sampling data which result from sampling the digital video signals VD.sub.o -VD.sub.k for each pixel. In contrast to this, in the case of an image reproduced on the CRT 5, although it is subject to sampling by a shadow mask, a video signal cannot be represented as thoroughly discrete images because of the high frequency characteristic of the CRT's electrical circuit typified by flyback transformers, the incapability of stopping down the spot diameter of scanning beam to a proper one, and other reasons. In other words, in representing an image the luminance of which is relative to the distance in a scanning direction, the luminance variation of an image reproduced on the panel display 7 results in stepped waveform, while that of an image reproduced on the CRT 5 results in rounded waveform without corners as shown in FIG. 6.
Therefore, it can be said that the luminance variation of an image reproduced on the CRT 5 can be expressed by a continuous function in which high frequency components are cut off in the spatial frequency characteristic of digital video signals toward the scanning direction.
Because of such difference in the spatial frequency characteristic of the input digital video signals VD.sub.o -VD.sub.k, in displaying characters and graphics, the panel display 7 can present satisfactory images with sharply outlined characters and graphics, while the CRT 5 displays images with somewhat blurred outlines. Further, in displaying characters and lines, the panel display 7, having independent pixels, can present sharply outlined characters and lines, while the CRT 5, poor in high frequency characteristic, results in images blurred in outline.
Meanwhile, natural pictures such as people and landscapes involve less edges in their luminance variation waveform and have more portions where the luminance at their outlines gradually varies. For this reason, in displaying natural pictures, the CRT 5, which has images blurred in outlines as described above, can offer satisfactory images, while the panel display 7, which involves stepped waveform of luminance variation, results in coarse display with the luminance variation emphasized in outlines.
In consequence, the panel display 7 is successful in character and graphic images but inferior to the CRT 5 in images of natural pictures.